The present invention relates to a force measurement cell, and in particular, to a force measurement cell suitable for use in an electromagnetic force balance type electronic balance.
An electromagnetic force balance type electronic balance generally has an electromagnetic force generator in which a movable coil is placed in a static electrical field generated by an electronic circuit, and the movable coil is attached to one end of a lever coupled with a tray. Also, the displacement of the lever due to a load to be measured on the tray is detected by a detector, the current flowing to the movable coil of the electromagnetic force generator is feedback-controlled such that the result of the detection always becomes zero, and the magnitude of the load to be measured is sought from the magnitude of the current necessary for zeroing the lever displacement.
The tray normally is supported by a Roberval mechanism which restricts displacement in the perpendicular direction in order to eliminate bias error. A Roberval mechanism has a structure in which a fixed part and a movable part are connected by two mutually parallel beams, above and below, each having flexible parts formed on both ends. The tray is attached to the movable part, and the movable part and the lever are connected by a flexible connecting member.
Among such electronic balances, one conventional type balance is constructed such that a block-like mechanism unit is formed by cutting out from a single piece of parent material a Roberval mechanism, a lever mechanism, and a connecting member connecting the Roberval mechanism and lever mechanism is used as a force measurement cell. The lever mechanism of the force measurement cell is connected to the movable coil of the electromagnetic force generator (see for example Japanese Utility Model Registration No. 2525318, hereinafter referred to as Patent Document 1).
In an electronic balance having a Roberval mechanism and a lever mechanism, in order to protect the beams of the Roberval mechanism and the movable members, such as the lever mechanism, from vibrations during transport and impact loads during use, and the like, conventionally, as exemplified in FIG. 7, it is common practice to provide a stopper 73 for restricting displacement of a lever 72 in the vicinity of the electromagnetic force generator 71. In the example shown in FIG. 7, a structure is adopted, in which a perpendicular pin 74 is attached to lever 72, and a hole 73a having a slight gap with respect to the outer perimeter surface of pin 74 is formed on stopper 73. By the protective mechanism using such stopper 73, the displacement of lever 72 in the horizontal direction is restricted to the gap between pin 74 and hole 73a. 
Also, among electronic balances using a force measurement cell consisting mainly of a block-like mechanism unit, there is known one which has a protective mechanism for preventing the lever mechanism, and the like, inside the force measurement cell itself from being damaged by vibrations and impact loads, in addition to a protective mechanism such as described immediately above (see for example Japanese Unexamined Patent Publication No. H10-132643, hereinafter referred to as Patent Document 2).
The protective mechanism disclosed in Patent Document 2 has a structure in which a bolt (safety bolt) extending in the perpendicular direction is fixed to the fixed part of the block-like mechanism unit. In addition, a through-hole allowing passage having a minute gap interposed with respect to the outer perimeter surface of the bolt is formed on the lever being the movable part as well as on the beams, and is constructed such that displacement of the lever mechanism and the like can be restricted to the gap between the through-hole and the bolt even when under the action of vibrations during transport and impact loads during use, and the like.
While the protective mechanism having a stopper for restricting lever displacement in the vicinity of the electromagnetic force generator, which is commonly used in conventional electronic balances, functions effectively against comparatively small loads such as vibrations during transport, the protective mechanism may not function effectively against great impact loads, such as falling, because the displacement of the movable parts such as the lever is restricted in only one location.
In an electronic balance using a force measurement cell consisting mainly of a block-like mechanism unit, it is preferable that displacement of the movable parts, such as the lever mechanism, be restricted in two locations by providing a mechanism for protecting the movable parts inside the force measurement cell as disclosed in the aforementioned Patent Document 2, in addition to a protective mechanism provided in the vicinity of the electromagnetic force generator as described above.
However, in the technology disclosed in Patent Document 2, because a comparatively long bolt is screwed into the block-like mechanism unit and must be run through with a prescribed slight gap of about 0.1 mm with respect to through-holes formed on the lever mechanism and the beams of the Roberval mechanism in multiple locations in the longitudinal direction of the bolt, disadvantageously high precision machining is required both for the through-holes formed on the block-like mechanism unit and for the bolt. Further disadvantageously, higher precision machining increase the cost.
The present invention has been made in consideration of the above, and the present invention is to provide the provision of a force measurement cell capable of restricting displacement of the movable parts inside the block-like mechanism unit based on an inexpensive construction.